The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is propelled by several turbojet engines each housed in a nacelle, each nacelle further accommodating a set of auxiliary actuating devices relating to its operation and ensuring various functions when the turbojet engine is in operation or shut down.
The modern nacelles are intended to accommodate a bypass turbojet engine capable of generating, via the blades of the rotating fan, a hot gas flow (also called primary flow) and a cold air flow (also called secondary flow) which circulates outside of the turbojet engine through an annular passage, also called flow path, formed between two concentric walls of the nacelle. The primary and secondary flows are ejected from the turbojet engine by the backside of the nacelle.
A turbojet engine nacelle generally has a tubular structure including:                a front section, or air inlet, located in front of the turbojet engine;        a mid-section, intended to surround the fan of the turbojet engine;        a rear section, intended to surround the combustion chamber of the turbojet engine and generally including thrust reverser means;        an ejection nozzle, whose outlet is located downstream of the turbojet engine.        
The rear section generally has an outer structure, which defines, with a concentric inner structure, called “Inner Fixed Structure” (IFS), the annular flow path used to channel the cold air flow.
The thrust reverser means allow, during the landing of an aircraft, improving the braking capability of said aircraft by redirecting forwards a major fraction of the thrust generated by the turbojet engine. In this phase, the thrust reverser generally obstructs the flow path of the cold flow and directs said cold flow forwardly of the nacelle, thereby generating a counter-thrust which is added to the braking of the aircraft wheels. The means implemented to carry out this reorientation of the cold flow vary according to the thrust reverser type. A common configuration is that of the thrust reversers called “cascade thrust reversers”. In this type of thrust reverser, the outer cowl of the rear section is sliding. The rearward translation of this sliding cowl allows uncovering thrust reverser cascades putting in communication the cold flow path and the outside of the nacelle. The translation of the sliding cowl further allows deploying blocking flaps in the cold flow path. Thus, by the combined action of the blocking flaps and the thrust reverser cascades, the cold flow is redirected forwardly of the nacelle.
As mentioned above, the thrust reverser means are housed in the rear section of a nacelle. Three types of structural configuration for the rear section are mainly known, namely the structures respectively called “C-duct” “D-duct” and “O-duct” structures.
In a D-duct structure nacelle, the inner and outer structures of the rear section of the nacelle are secured to each other, via two connecting islets called bifurcations. The bifurcations are disposed respectively according to the positions called “twelve o'clock” position (upper bifurcation) and “six o'clock” position (lower bifurcation). It should be recalled that the “twelve o'clock” and “six o'clock” positions are conventionally defined by analogy with a watch dial, the nacelle being in the operation position, that is to say under the wing. The “twelve o'clock” position is accordingly located at the attachment mast of the nacelle, while the “six o'clock” position corresponds to the diametrically opposite position. In the case of a D-duct structure nacelle, the sliding cowl is mounted in translation on the outer structure of the rear section. The sliding cowl is generally constituted of two half parts.
In an O-duct or C-duct structure nacelle, the rear section is configured such that a lower bifurcation is not necessary. This represents a great gain in efficiency for the propulsion unit since the cold flow path is no longer obstructed in its lower part as is the case for the D-duct structures. Furthermore, the O-duct or C-duct structures also allow significant gains in terms of mass.
In an O-duct or C-duct structure, the sliding cowl, or movable cowl, is generally mounted on slides disposed on either side of the suspension pylon (or mast) of the propulsion unit. These slides may be disposed directly on the pylon, or on an intermediate member secured to the pylon when the propulsion unit is mounted. The cowl is guided and supported only at these slides, therefore only in the vicinity of the “twelve o'clock” position.
Moreover, in an O-duct structure, the sliding cowl forms a one-piece structure. In order to meet various constraints such as avoiding the re-ingestion of air by the motor, avoiding directing a fraction of the thrust towards the fuselage of the aircraft, etc., the profile of the cascades ensuring the redirection of the cold flow, is generally not uniform along the circumference of the cascade assembly. It follows therefrom that the lateral efforts associated with the thrust reversal undergone by the cascades are not uniformly distributed. The sum of these lateral efforts is non-zero, which generates a lateral force applied on the cascade assembly and therefore on the propulsion unit. This lateral force generates a pendulum movement of the propulsion unit.
This non-uniform distribution also generates a non-uniform deformation of the thrust reverser, which become oval during the thrust reversal phases.